1. Field of the Invention
This invention relates to solid electrolytic capacitors of the type which make use of conductive polymers as a solid electrolyte and which have good frequency characteristics and high reliability under high temperature conditions. The invention also relates to a method for manufacturing the capacitors of the type mentioned above.
2. Description of the Prior Art
Recent trends toward digitalization in the electric and electronic fields have demanded capacitors which are small in size and large in capacitance with a low impedance in a high frequency range.
Known capacitors which have been used in high frequency ranges include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. These capacitors are disadvantageously so large in size that a large capacitance is difficult to obtain.
On the other hand, a certain type of electrolytic capacitor is known as having a large capacitance. This type of capacitor includes, for example, an aluminum dry electrolytic capacitor and an aluminum or tantalum solid electrolytic capacitor. These electrolytic capacitors are advantageous in that since an anodized film serving as a dielectric layer can be formed very thinly, a large capacitance can be realized. However, the anodized film is liable to undergo damages, so that it becomes necessary to provide an electrolyte between the anodized film and a cathode in order to repair the damages.
With aluminum dry electrolytic capacitors, anode and cathode aluminum foils which have been, respectively, etched, are convolutely wound through a separator and a liquid electrolyte is impregnated in the separator. This presents the problems that since the liquid electrolyte is ion conductive in nature with a large specific resistance, so that the loss (tan .delta.) becomes great with very poor frequency and temperature characteristics. In addition, the leakage and evaporation of the liquid electrolyte inevitably occurs, thus leading to a decrease of the capacitance and an increase of the loss with time.
With the tantalum solid electrolytic capacitor, manganese dioxide is used as the electrolyte. Accordingly, the problems on the temperature characteristic and the changes of the capacitance and loss in relation to the time can be overcome. However, the relatively high specific resistance of manganese dioxide results in the loss and a frequency characteristic of impedance poorer than those of the layer-built ceramic capacitors and film capacitors.
In order to solve the above problems, there have been recently proposed solid electrolytic capacitors wherein highly conductive polymers, which contain an anion of a support electrolyte as a dopant and which are obtained by electrolytically polymerizing heterocyclic monomers such as pyrrole, thiophene and the like, are formed on an anode through a conductive underlying layer (Japanese Kokai Patent Application NOs. 60-37114 and 60-244017). The solid electrolytic capacitor has good frequency and temperature characteristics.
However, in the known solid electrolytic capacitors which make use of the conductive polymer as the electrolyte, the dopant used is, for example, BF.sub.4.sup.-, ClO.sub.4.sup.-, PF.sub.5.sup.-, AsF.sub.5.sup.-, ammonium borodisalicylate or the like. At high temperatures, these dopants are liable to be de-doped or part of the polymer structure is oxidized, so that the length of the conjugated bonds is reduced. This results in a lowering of the electric conductivity. This undesirably leads to a lowering of the capacitance and an increase of the loss (tan .delta.). In addition, not only electric characteristics, but also mechanical strength and adhesiveness of the conductive polymer layer are degraded when the solid electrolytic capacitor is placed under high temperature conditions over a long term